1. Field of the Invention
The present invention relates to a semiconductor optical device, in particular, the invention relates to a device that suppresses, what is called, the double injection phenomenon.
2. Related Prior Art
FIG. 5 illustrates a typical light-emitting device applicable in the optical communication system. The device 100 shown in FIG. 5 comprises, on an InP substrate 102, an n-type InP cladding layer 104, an InGaAsP active layer 106, a p-type InP cladding layer 108, and a p-type GaInAs contact layer 110, where the layer configuration is called as the double hetero (DH) structure. These semiconductor layers constitutes a mesa 112 extending along the primary surface of the substrate 102, and a semi-insulating InP 114 doped with iron (Fe) buries the mesa 112 in both sides thereof. On the InP 114 is formed with an insulating film 16. The electrode 118 comes in contact with the contact layer 110 within an opening 116a of the insulating film 116, while, the other electrode 120 is formed on the whole back surface of the InP substrate 102. Various prior arts, such as Japanese Patent application published as H05-183229A, have disclosed such a light emitting device.
The burying region 114 generally dopes iron (Fe) or ruthenium (Ru), which causes deep acceptor levels, and region 114 shows the semi-insulating characteristic by trapping electrons in these deep acceptor. However, in the device has a function of a laser diode (hereafter denoted as LD) shown in FIG. 5, the semi-insulating burying region 114 comes in physically contact with the p-type InP cladding layer 108. Because this p-type InP cladding layer 108 has substantial thickness to confine the light generating in the active layer 106, where the prior art mentioned above has a thickness of 1.5 μm for the p-type cladding layer, the contact area between the burying region 114 and the p-type cladding layer 108 becomes broader; accordingly, holes in the p-type cladding layer 108 easily diffuses into the burying region 114. Holes in the burying region 114 may recombine with the electrons trapped in the deep acceptors, which makes the acceptor levels vacant; and the electrons in the n-type cladding layer 104 moves into the burying region 114 and be trapped by vacant deep acceptors.
The mechanism explained above, which is called as the double injection phenomenon, causes the leak current in the LD to degrade the emission efficiency.
The prior art mentioned above forms an n-type layer between the p-type cladding layer 108 and the semi-insulating burying region 114 to trap holes from the p-type cladding layer 108. However, when this n-type layer comes in contact with the n-type cladding layer or the n-type substrate, electrons may flow in this n-type layer to the n-type cladding layer and the n-type substrate so as to bypass the active layer, which also increases the leak current and degrades the emission efficiency. Thus, such an n-type layer must be restricted in just sides of the p-type cladding layer, but the process to form such an n-type layer becomes quite hard.